The next generation of solid-state lighting is seeking to provide advances in brightness, efficiency, colour, purity, packaging, scalability, reliability and reduced costs. The creation of light emitting devices from silicon based materials, upon which the modern electronic industry is built, has been the subject of intensive research and development around the world. To overcome the inherent low efficiency of light emission from indirect bandgap materials, such as bulk silicon and other group IV semiconductor materials, extensive research has been directed to nanostructures, i.e. nano dots, nanocrystals and superlattice structures, and materials comprising silicon nanocrystals and/or other luminescent centres, such as rare earth ions, in a suitable host matrix.
With reference to FIG. 1, a simple electroluminescent light emitting device 1 comprises a conductive substrate 2 which acts as a first electrode, e.g. an N+ silicon substrate, on which an active layer 3, i.e. light emitting layer, of a suitable thickness is deposited. The active layer comprises luminescent centers, which may be electrically excited, e.g. rare earth oxides and/or nano-particles in a host matrix such as silicon dioxide. A second electrode is deposited on the active layer, preferably a transparent conducting oxide (TCO) layer, typically indium tin oxide (ITO), to allow for light extraction from the active layer. Subsequently, a metallization step provides ohmic electrical contacts 5 and 6 to the first electrode (substrate) 2 and the second electrode (TCO) layer 4, respectively, to provide for application of an electric field and injection of electric current into the active layer 3 for excitation of light emission. At low electric fields there is no current flow and the structure behaves as a capacitor. With the application of an electric field above a characteristic threshold field, electrons can be injected into the active layer 3 from either the N+ substrate 2, via contact 6, or the ITO electrode 4, via contact 5, depending on the direction of bias. In the ballistic regime, electrons gain energy from the electric field in proportion to the distance traveled, and electrons may gain sufficient energy to excite luminescent centres, e.g. by impact ionization or impact excitation. Nevertheless, a simple device structure of this type, with a single relatively thick active layer, e.g. 150 nm, tends to have poor luminous efficacy, low excitation efficiency and low brightness.
Related co-pending applications, now published as U.S. Patent Publications Nos. 2007/0181898, entitled “Pixel Structure for a Solid-State Light Emitting Device”; 2008/0093608, entitled “Engineered Structure for Solid-State Light Emitters”; and 2008/0246046, entitled “Pixel Structure for a Solid-State Light Emitting Device” disclose emitter layer structures and methods for fabricating light emitting devices using active (light emitting) layers comprising luminescent centres in a host matrix material comprising a wide bandgap semiconductor or dielectric material, and respective drift layers adjacent each active layer, comprising a dielectric or wide bandgap semiconductor material. For example, active layers may comprise rare earth oxides or rare earth luminescent centres in a dielectric host matrix material such as silicon dioxide or silicon nitride, which may be electrically excited to produce visible light emission. For efficient excitation, corresponding drift layers adjacent each active layer have a thickness, relative to an applied electric field, dependent on a respective excitation energy of an adjacent active layer. Further examples of suitable materials for active layers and drift layers in such a structure are disclosed in related co-pending U.S. Patent Application Publication no. 20100032687, entitled “Engineered Structure for High Brightness Solid-State Light Emitters. Beneficially, emitter layer structures may comprise multiple thin active layers, which may emit different wavelengths that are combined to provide light emission of a desired colour or CRI (colour rendering index). Preferably, in deposition of the emitter layer structure, careful control of the thickness of active layers and drift layers, and in particular control of the thickness of each drift layer, dependent on the operational electric field and required excitation energies of respective adjacent active layers, provides for improved excitation efficiency.
For applications requiring high brightness, e.g. solid-state lighting, this multilayer engineered light emitting device structure offers advantages in terms of efficiency, brightness, colour control and lifetime compared with a conventional thin film electroluminescent device (TFEL). Furthermore, these structures may be fabricated using materials based on or compatible with silicon or other Group IV semiconductors.
On the other hand, whether an emitter layer structure comprises a single active layer 3 as shown in FIG. 1, or a multilayer engineered light emitting structure as described in the above referenced copending applications, another drawback of a simple light emitting structure such as shown in FIG. 1, is that under the high electric fields needed for operation, planar breakdown of the active layer 3 at the edge of the light emitting device 1 will dominate and limit the electric field that can be applied. In particular, as discussed in the above-mentioned copending applications, propagation breakdown may occur in large area emitter structures, and a high internal electric field in the vicinity of the contact interfaces with the active layer 3, may lead to an early breakdown and destruction of the light emitting device 1.
Therefore, further improvements in structures and processes for fabrication of solid-state light emitting devices are desirable, particularly for applications requiring higher brightness, luminous efficacy, and improved reliability, such as solid-state lighting.
An object of the present invention is to provide improved device structures and processes.